Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography. Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In the dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated, for example, by a thermal printer head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
In recent years, various printers allowing higher-speed printing have been developed and commercialized increasingly in the field of the dye-diffusion transfer recording systems. The high-speed printing is a performance desirable for shortening the time of the user waiting for printing in photo shop.
In order to prevent thermal sticking between a thermal printer head of a printer and a heat-sensitive transfer sheet and to give the thermal printer head and the ink sheet a slipping property therebetween, a heat-resistant lubricating layer is formed on the heat-sensitive transfer sheet surface contacting the thermal printer head. The thermal sticking may cause a break of the heat-sensitive transfer sheet when an image is printed. On the other hand, in a case where the slipping property is insufficient, the heat-sensitive transfer sheet may be stretched or creased, or deformed into some other form when an image is printed. As a result, an image defect may be caused. According to high-speed printing, a thermal printer head comes to contact the heat-resistant lubricating layer at a higher temperature and a higher speed. Thus, the heat-resistant lubricating layer is desired to have even better performances.
For example, JP-A-8-90942 (“JP-A” means unexamined published Japanese patent application) discloses that phosphoric acid ester series surfactants are added to the heat-resistant lubricating layer in order to improve a lubricating (slipping) property. Further, Japanese Patent No. 2655544 discloses that zinc salts of specific phosphoric acid ester are added to the heat-resistant lubricating layer to give a lubricating property.
Ordinarily, these heat-sensitive transfer sheets are stored as such a product form that the heat-sensitive transfer sheet is prepared by coating a dye layer on or above a base film, followed by drying, and then once stored as a roll form, and subsequently the heat-sensitive transfer sheet is taken out from the roll and cut into a sheet having a desired width, and then rewound in a roll form which is then set in a printer.
Accordingly, in this product form, the heat-resistant lubricating layer and the dye layer are left to stand over time in the state that these layers contact each other. For this reason, a dye of the dye layer is transferred to the heat-resistant lubricating layer over time, and thereafter if a printing is performed, the heat-resistant lubricating layer with a dye adhered thereto is heated with a thermal head at the time of print. Therefore, if the printing is continued in large numbers, thermally decomposing materials of the dye accumulate as a stain of the thermal head, finally, the materials eventually causes a problem of viewing surface defect at the time of print.
On the other hand, the study of the heat-resistant lubricating layer has been previously carried out. For example, as mentioned above, JP-A-8-90942 describes that phosphoric acid ester-series surfactants are added to the heat-resistant lubricating layer in order to improve a lubricating property. Further, Japanese Patent No. 2655544 discloses that zinc salts of specific phosphoric acid ester are added to the heat-resistant lubricating layer to give a lubricating property. Further, the study of the dye that is used in the dye layer has been previously carried out. For example, in JP-B-6-19033 (“JP-B” means examined Japanese patent publication), the study of a yellow dye having a specific structure is disclosed.
However, the heat-sensitive transfer sheets described in these patent literatures are not necessarily satisfied to resolve the aforementioned problems. Therefore, improvement of the heat-sensitive transfer sheets has been earnestly desired.
Further, various methods whereby a high-quality image can be obtained are previously proposed. For example, Japanese Patent No. 3596922 discloses a specific dye having high transferability (high-transferable dye) whereby a high density can be obtained. However, a problem arises such that a scumming owing to a kickback is likely to occur as a result of using the high-transferable dye. Herein, the term “kickback” is such a phenomenon that, during storage of the roll-formed heat-sensitive transfer sheet produced by coating a dye layer on or above a base film, the dye transfers to a heat-resistant lubricating layer (this step is called “kick”), and when the heat-sensitive transfer sheet is rewound in order to process it into a product form, the dye transferred to the heat-resistant lubricating layer transfers back to the dye layer or a protective layer (this step is called “back”). If the dye layer or the protective layer is stained owing to the kickback, image quality conspicuously deteriorates due to change of color hue and scummimg of a white background. Therefore, improvement of the heat-sensitive transfer sheet has been earnestly desired.
As to the technique of resolving the kickback problem, for example, JP-A-2003-154763 discloses that a compound capable of chelate-reacting with a thermally transferable dye is contained in a back layer. However, this compound is decomposed by heat at the time of printing, and the decomposed material accumulates as a stain of the thermal printer head. As a result, this compound tends to cause such a problem that viewing surface defects occur. Therefore, another technique for resolving this problem has been required.